The art of separating solid particles from fluid streams by passing a fluid through a screening device having perforations of such size that solid particles are retained on the screen surface instead of passing through is well known. In some processes however, it is not simply a matter of passing solid particles carried in a fluid stream across a screening device to obtain a separation. Some fluid streams contain additional materials that cause solid particles to adhere to each other or to the screen surface, ultimately bridging the screen openings and blinding the screen. Such a problem is frequently encountered in the drilling of wells during which operation a drilling mud is circulated into the bore hole.
During well drilling operations, drilling mud is continuously circulated between the bore hole and the surface. The mud removes drilling cuttings, also called drilled solids, from the face of the drill bit in the bore hole and carries them to the surface. Operations at the surface separate the cuttings from the mud. The mud is then recirculated to the drilling operation. The drilling mud must be maintained as clean and free of contamination by cuttings and foreign materials as possible. The drilling mud is typically cleaned by several types of equipment sequentially, including vibratory screening machines commonly referred to as shale shakers.
Frequently in drilling operations a gummy clay formation is encountered which makes the separation of drilling cuttings from the drilling mud by screening operations very difficult The gummy clay, sometimes called gumbo, adheres to the surfaces of screens and screening machines causing a buildup which can blind the screens so that the drilling mud, rather than passing through the screen where it can be recirculated, passes over and off the end of the screen where it is unrecoverable. These problems are at least partially due to the non-Newtonian rheological properties of the gumbo, which is largely comprised of hydrated clay particles. In particular, gumbo generally displays the properties of a rheopectic fluid. That is, it shows an increase in its viscosity with time under the influence of a suddenly applied constant shear stress.
Because the gumbo behaves as a rheopectic fluid, it tends to form into a cohesive mass on the screening equipment, sometimes referred to as a gumbo patty. Gumbo patties resist separation by vibratory screening equipment because they do not readily pass through the screen openings. Further, gumbo patties can not be conveyed off the screen to a waste stream because their viscosity increases as additional energy is applied by the screening machine in an attempt to transport them off the screen. This increase in viscosity results in the gumbo becoming more resistant to conveying as more energy is applied to transport it off the screen.
Thus gumbo tends to reduce the capacity of mud cleaning equipment to separate cuttings from drilling mud by clogging the screens and reducing their open area. As more gumbo is removed during well drilling, it tends to accumulate in the mud screening equipment. This obviously her reduces the ability of the screening equipment to clean the drilling mud. Eventually, this accumulation threatens the ability of the equipment to effectively clean the recirculating mud and may even force reduction of the drilling rate, with disastrous effects on drilling costs. In extreme cases, the well must be relocated, drilled in a different direction, or abandoned entirely.
The cost of drilling mud represents a substantial portion of the total cost to drill a well. Contamination of the mud can therefore easily increase drilling costs to a prohibitive level. Further, reducing the capacity of the mud cleaning equipment can limit the rate at which a well can be drilled. This increases the time required for drilling, again increasing drilling costs. Additionally, contamination of the drilling mud by gumbo or drilling cuttings increases the viscosity of the mud, which reduces the drilling rate and increases drilling costs.
Previous attempts to solve problems caused by gumbo have met with little success. Many of the prior efforts have focused on chemical additives in attempts to change the properties of the drilling mud, which add cost to an already substantial drilling expense. Mechanical approaches, such as increasing the vibrational amplitude and adding various types of water sprays have proven ineffective because the gumbo simply forms a new steady state gumbo patty at the new operating conditions. Other strategies have included applying specialized coatings to the screens, such as are described in U.S. Pat. No. 3,963,605. Such coatings add cost to the screening equipment, and are quickly worn off the screens by abrasive cuttings. Accordingly, there remains a need in the art for apparatus to remove gumbo from recirculating drilling mud. It would be preferable if such apparatus could be used in conjunction with, and upstream of, existing mud cleaning equipment and removed gumbo from the drilling mud before drilling cuttings were removed from the mud.